We determined the crystallization region of KGd(PO
3)
4 (KGP) in the ternary system Gd
2O
3-K
2O-P
2O
5, the isotherms of saturation temperature and six neighboring phases: KPO
3, GdPO
4, KGdP
4O
12,Gd(PO
3)
3-orthorhombic, Gd(PO
3)
3-monoclinic, and GdP
5O
14. We successfully grew KGP single crystalsby the top-seeded solution growth (TSSG) slow cooling method. To optimize the growth process andobtain high-quality KGP crystals, solution composition and seed orientation were analyzed and a growthdevice was added. KGP is isostructural to KNd(PO
3)
4, crystallizes in the monoclinic system
P2
1 and thecell parameters determined are
a = 7.2701(11),
b = 8.3818(8), and
c = 7.9608(10) Å,
= 91.776(12)
,
Z = 2, and
V = 484.87(11) Å
3. We studied how they evolved as the temperature increased from roomtemperature to 773 K and observed an almost isotropic linear thermal expansion, the tensor of which wealso calculated. In the room temperature to 1273 K range analyzed, we found that KGP irreversiblydecomposed at 1142 K. The second harmonic efficiency, qualitatively measured by the Kurtz method,was at least similar to that of KH
2PO
4 (KDP).